Suppressing short channel effect in a NAND-type flash memory cell may be increasing its effective channel length by increasing impurity concentration in a substrate while reducing impurity concentration in diffusion layers in the cell.
However, reduction in impurity concentration in diffusion layers will increase a resistance value in a NAND string, and thus lower the saturation current in each cell. This increases a voltage difference between on/off discrimination currents and thus increases time required for each reading operation.
The conventional NAND-type flash memory may have a problem that it is difficult to suppress short channel effect while reducing a voltage difference between on/off discrimination currents therein.